1. Field of the Invention
The field of the invention is chat of digital cellular mobile radio systems such as those conforming to the GSM public land mobile radio network standard.
The expression "GSM standard" here means not only GSM standard 900 covering GSX mobile radio systems operating in the 900 MHz band but also the DCS 1800 standard covering systems operating in the 1800 MHz band.
To be more precise, the invention concerns a particular BCCH carrier structure and a method of measuring the signal level received by a mobile station on a BCCH carrier of this kind.
2. Description of the Prior Art
A digital cellular mobile radio system is generally implemented within a network of geographical cells through which mobile stations travel. A base station is associated with each cell and a mobile station communicates via the base station associated with the cell in which it is located.
Each base station (and therefore each cell) uses one or more pairs of radio carriers, necessarily including a pair of BCCH carriers specific to it The carriers of the same pair are respectively used for the uplink (mobile station to base station) and downlink (base station to mobile station) directions.
In the conventional way, each carrier is segmented temporally using a fixed time-division multiple access (TDMA) scheme. The time axis is divided into successive frames of fixed duration each divided into a particular number of time slots, the recurrence of a particular time slot in each frame constituting a physical channel onto which a plurality of logical channels can be multiplexed.
The uplink BCCH carrier (transmitted by the mobile station) generally supports a logical traffic channel (TCH) which is used to transmit user data or speech and a logical random access channel (RACH) which a mobile station uses to access the network in order to log on in a cell or to make a call.
The downlink BCCH carrier (transmitted by the base station) supports one or more traffic channels multiplexed onto one or more physical channels and the following signaling channels, which are generally multiplexed onto a physical channel consisting of the recurring first time slot of each frame:
a logical broadcast control channel (BCCH) which supplies to all mobile stations general information on the network, the cell in which the mobile station is located and the adjacent cells, PA1 a logical synchronization channel (SCH) which carries information for synchronizing frames and identifying the base station transmitter, PA1 a logical frequency channel (FCH) which provides information on the carrier used, PA1 a logical access grant channel (AGCH) which is used to assign dedicated resources (signaling channel (SDCCH) or traffic channel) to the mobile station requesting them via the random access channel, and PA1 a logical paging channel (PCH) which is used to locate a mobile station and to advise it of a call from the network. PA1 a half-rate traffic channel, PA1 a slow dedicated control channel, PA1 a synchronization channel, PA1 a frequency channel, PA1 an access grant channel, and PA1 a paging channel. PA1 on the one hand, always at the same time slot position (the first time slot in each frame, for example, as in the GSM standard) and with a particular timing, and PA1 on the other hand, at different time slot positions depending on the frame (it is then as if the logical BCCH appeared to slip. between BCCH time slots,. i.e. between different physical BCCH, from one frame to another. PA1 dividing the monitoring window or each of the N.sub.F successive monitoring window(s) into a plurality of time portions so that, on receiving the BCCH carrier, at least one of the time portions contains only a part of the BCCH time slot or one of the N.sub.R BCCH time slots, PA1 measuring the received signal level in each time portion of each of the N.sub.F successive monitoring windows, and PA1 comparing the measured received signal levels in order to detect the maximal received signal level, which is deemed to constitute the signal level received on the BCCH carrier.
The present invention is specifically concerned with the downlink BCCH carrier which in the remainder of the description is simply referred to as the BCCH carrier.
An essential role of the BCCH carriers is to enable the system to determine the cell in which each mobile station is located so that it can detect when a mobile station moves from one cell to another. A mobile station in a given cell, known as the current cell, monitors the BCCH carriers of adjoining cells continuously and sends the corresponding measurements to the system so that it can determine if the mobile station is moving from one cell to another.
During a call, monitoring of adjoining cells by the mobile station enables the system to decide when handover must take place, i.e. when an adjoining cell must become the new current cell.
Similarly, on standby, monitoring of adjoining cells by the mobile station enables the system to detect a change of location area identity (LAI).
In the conventional way a mobile station monitors the BCCH carriers of adjoining cells by measuring the signal level (i.e. the power) received on each BCCH carrier. The adjoining cell whose identifier has been decoded and whose BCCH carrier is received with the maximal power is then chosen as the new current cell.
At present each base station transmits its BCCH carrier at constant power. It is generally accepted that all time slots of all frames of the same BCCH carrier must be transmitted with the same power. In other words, all physical channels (themselves carrying multiplexed logical channels, i.e. logical signaling channels and traffic channels) are transmitted with a maximum power.
Transmitting a BCCH carrier with a constant power has the major drawback of implying a high level of interference on the BCCH carrier. Consequently, re-use of BCCH carriers in the cells is low. In other words, it is difficult to use the same BCCH carrier again in other cells and the frequency band needed to assure a given traffic is therefore greater, which constitutes a non-negligible economic drawback.
Also, it is not possible to apply to a constant power BCCH carrier interference reduction techniques such as power control or discontinuous transmission (DTX).
An objective of the invention is to alleviate these various drawbacks of the prior art.
To be more precise, one objective of the present invention is to provide a BCCH carrier that can be transmitted at non-constant power, in particular to reduce the level of interference on the BCCH carrier and to increase re-use of BCCH carriers in the various cells.
Another objective of the invention is to provide a monitoring strategy suitable for a BCCH carrier of the above kind transmitted at non-constant power In other words, another objective of the invention is to provide a method of measuring the signal level received by a mobile station on a BCCH carrier of the above kind.